Recent evidence suggests that the decline in old age of the levels of circulating thymic hormones may contribute to the decline in immune competence and increase in autoimmune disorders characteristic of senescence. The main objective of my proposed investigation is to learn what roles are played by two well-characterized thymic hormones, FTS and thymopietin, in regulation of the primary immune response of mice to sheep erythrocytes and to soluble hapten-protein conjugates. The approach chosen emphasizes the use of anti-Lyt alloantisera to simplify the contribution of specific T cell subsets to the hormonal effects observed. Experiments are proposed to examine the effects of these agents on T helper and suppressor activity, as well as on the role of T helper signals in producing feedback inhibition. Insight into the effects of thymic hormones on the normal immune response ought to help us to design experiments to analyze the role played by decreased thymic hormone levels in immune senescence.